1. Field of the Invention
The present invention relates to a rolling method and apparatus for continuously reducing the outer diameter of a hollow tube of carbon steel, stainless steel, or the like in a cold state, and to a method and apparatus for die processing, in addition to continuous cold reducing or cold stretch reducing.
2. Description of the Related Art
Hot stretch reducing is known as a method of producing metal tubes. In this method, a plurality of stands having three rolls forming arcuate grooves are disposed in tandem, and a heated mother tube is passed through the stands, so that the outer diameter of the mother tube is continuously reduced. Since this method is hot rolling, it involves its problems in the dimensional precision of the products and surface quality, and it is expensive due to the need for a heating furnace and fuel and other expense.
In producing of metal tubes, when a tube with small diameter of less than an inch is produced, generally, a hollow mother tube produced by hot rolling is acid-cleaned, lubricated, and then cold drawn by die or cold rolled by Pilger rolling mill.
FIG. 1 is a schematic side view showing the constitution of an apparatus in case of producing tubes by the cold drawing method. In the drawing, numeral 21 designates a tube, and the tube 21 is inserted into a die 22 having a circular hole. At the exit side of the die 22, a drawing machine 23 is disposed at a specified interval., and it is designed to draw a small diameter tube to reduce it in diameter. At this time, a chuck 24 disposed between the die 22 and the drawing machine 23 holds the small diameter tube. For this holding, as pretreatment of the drawing, a step for squeezing to reduce one end of the tube 21 is required. In drawing, a large tension is applied to the tube, but this tension must be limited to an extent that the mother tube may not be broken, and the reduction rate in one pass is limited, and when the total reduction rate becomes higher, moreover, the mother tube undergoes work-hardening and therefore intermediate annealing is required, which results in low yield and low working efficiency.
In the latter method of cold rolling, on the other hand, a pair of rolls having grooves tapered along the circumference are used and the tube is reduced in diameter and processed by moving the rolls reciprocally while pressing down by holding the tube by the rolls. In this cold rolling method, the reduction rate of mother tube in one pass is greater than the former method, but the working efficiency is inferior because the rolls must be moved reciprocally and pressed down upon the tube.
In producing small diameter tubes, the hot stretch reducing method mentioned above may be employed in certain cases, and the yield and working efficiency are notably enhanced by the hot stretch reducing method, but, as mentioned above, there are problems in the dimensional precision of products and surface quality. It also is expensive due to a need for a heating furnace and fuel and other expenses.
Accordingly, as disclosed in the Japanese Patent Application Laid-open 63-33105 (1988) and the collected papers of the 118th general meeting of Iron and Steel Society of Japan (CAMP-ISIJ, vol. 2, 1989, pp. 1494), the three-roll type cold stretch reducing method applying the hot rolling in cold rolling has been proposed.
FIG. 2(a) is a schematic side view explaining the arrangement of stands of a stretch reducer, and FIG. 2(b) is a schematic front view explaining the arrangement of stands of the stretch reducer. In the drawings, numeral 25 designates rolls, and a plurality of stands 26, 27, 28, . . . having three rolls 25, 25, 25 disposed at intervals of 120 degrees around pass line X are disposed in the pass line direction. The stands are arranged in tandem, by matching the calibers, varying the phase of the roll disposition of the adjacent stands by 60 degrees, and decreasing the caliber diameters gradually. In the final stand, a stand of round caliber is disposed.
Stand calibers consist of round calibers and oval calibers. FIGS. 3(a), (b) are sectional views showing the calibers used in the three-roll stretch reducer, and specifically FIG. 3(a) shows the round caliber, and FIG. 3(b), the oval caliber. The round caliber is a caliber composed of an arc R.sub.1 having the center in the caliber center, and the oval caliber is a caliber having another arc R.sub.2, with the center of the arc located on the center line of the roll gap, in the relief part of the caliber.
Among stands having oval calibers and round calibers consisting of three rolls, while applying a tension to the tubes between stands by setting the peripheral velocity ratio of roll surface between the adjacent stands larger than the elongation rate of tubes in a single stand, the mother tube is continuously passed among stands to reduce it to desired outer diameter.
In such three-roll cold stretch reducing method applying hot rolling in cold rolling, the tube wall thickness increases or decreases in the circumferential direction due to the reason mentioned below to cause so-called wall thickness deviation, and the inner sectional shape of the tube is deformed into a hexagonal form as shown in FIG. 4. That is, in hot stretch reducing, the friction coefficient of roll and mother tube is 0.3, and a sufficient tension is obtained among stands, and increase of wall thickness being a cause of wall thickness deviation is sufficiently suppressed, and deviation hardly occurs, but in cold stretch reducing, the friction coefficient is less than 0.1, being less than 1/3 of that of hot process, and sufficient tension cannot be obtained among the stands, and the increasing tendency of uneven wall thickness in the tube peripheral direction cannot be suppressed between the abutting portions of the roll groove bottom and roll groove edge.
Besides, seizure of the mother tube to the roll is caused by slipping by reason that the tension among stands is increased, or overfilling of mother tube into the roll gap occurs by reason that specified tension is not obtained.
To solve such problems, a method of rolling by setting the groove bottom diameter of the roll at 10 times or more of the outer diameter of the mother tube has been disclosed in the Japanese Patent Application Laid-open Hei. 4-4905.
FIG. 5(a) and FIG. 5(b) are conceptual diagrams explaining the ratio of outer diameter of mother tube and diameter of roll groove bottom, and rolling condition of mother tube, being a front view of roll and a side view of roll, respectively. Referring to these drawings, the method disclosed in the Japanese Patent Application Laid-Open 4-4905 is explained. Three rolls 31, 32, 33 are disposed around the pass line of mother tube A whose distance from central axis C.sub.2 to the outer circumference is D.sub.2 /2, and the groove bottom radius of these rolls, that is the distance from the axial center C.sub.1 to the groove bottom is D.sub.1 /2. By using the rolls 31, 32, 33 whose D.sub.1 /D.sub.2 is 10 or more, the frictional force is enhanced, and the outer diameter of the mother tube A is continuously reduced in cold state.
In the conventional method as mentioned above, incidentally, since the contact area of the roll and mother tube is increased by setting the roll groove diameter at more than 10 times the outer diameter of the mother tube, a sufficient frictional force can be obtained even in the cold stretch reducing method being low in the coefficient of friction, so that a necessary tension among rolls is obtained. However, increase of contact area gives rise to increase of rolling force, that is, rolling load, and the required power for rolling and torque increases, and the increase of roll groove bottom diameter causes to increase the roll volume and gives rise to a substantial enlargement of facility, and problems of economy and facility are involved, and moreover in the three-roll rolling method, overfilling of tube into roll gap is likely to occur, and the reduction per stand cannot be increased, and therefore the rolling efficiency is poor, the number of stands required for reducing a tube to specified outer diameter increases, and the facility becomes gigantic.